As a conventional technique, for example, Japanese Unexamined Patent Publication No. 116438/1998 (Tokukaihei 10-116438, published on May 6, 1998) discloses a method for detecting inclination of an object lens in an optical pickup device. The method is discussed referring to FIG. 16.
In FIG. 16, an object lens 101 has a curved surface (lens functioning section) 101a acting as a lens, and a ring-shaped plane surface 101b at the circumference. The normal direction of the plane surface 101b virtually conforms to an optical axis direction of the object lens 101.
Light is emitted from a light source 102, is transmitted through a pin hole 103, which is disposed ahead of the light source 102, is converted into a parallel light pencil through a collimate lens 104, is reflected on a beam splitter 105, and is emitted into the object lens 101.
Light emitted into the object lens 101 is partially reflected on the curved surface 101a and the plane surface 101b and the rest passes through the object lens 101. In this case, the reflectivity depends upon a refractive index of a material forming the object lens 101, and an entry angle of light. Normally, light has a small reflectivity of several percents.
Light reflected on the curved surface 101a and the plane surface 101b is transmitted through the beam splitter 105, and is directed to a light-receiving element 107 through the collimate lens 106. A CCD and the like is used as the light-receiving element 107.
Light reflected on the plane surface 101b forms a condensing spot on a point of the light-receiving element 107, and light reflected on the curved surface 101a forms a spot expanding around the condensing spot. Here, a position of a condensing spot on the light-receiving element 107 is detected so as to detect the inclination of the object lens 101. The condensing spot is formed by light reflected on the plane surface 101b. This conventional arrangement detects the inclination of the object lens 101 by detecting on the light-receiving element 107 a condensing spot position of light which is reflected from the plane surface 101b. However, on the light-receiving element 107, a light spot is formed by the curved surface 101a around a condensing spot formed by the plane surface 101b, so that it is difficult to separate the condensing spot of the plane surface 101b and the spot of the curved surface 101a, resulting in deterioration in accuracy of detecting a position of a condensing spot formed by the plane surface 101, namely, accuracy of detecting the inclination of the object lens 101.
Moreover, in view of light reflected from the beam splitter 105 as well, the accuracy of detecting is further deteriorated. To be specific, light reflected from a surface of the beam splitter 105 is also emitted onto the light-receiving element 107; however, the beam splitter 105 also has a plane surface, so that a condensing spot is formed on a point of the light-receiving element 107. The reflectivity on the surface of the beam splitter 105 is virtually the same as that of the plane surface 101b of the object lens 101; thus, it is further difficult to separate the condensing spots on the light-receiving element 107.
In order to solve the above problem, Japanese Unexamined Patent Publication No. 59172/1994 (Tokukaihei 6-59172, published on Mar. 4, 1994) discloses that the plane surface 101b has a reflecting part for increasing reflected light.
However, with this arrangement, it is difficult to, for example, sufficiently eliminate the influence of the curved surface 101a and to detect the inclination of lens with high accuracy of detecting.
Further, for example, a combination of a plurality of lenses is also applicable for the object lens; however, Japanese Unexamined Patent Publication No. 116438/1998 does not disclose inclination detection of a lens having such an arrangement.